1. Field of the Invention
The present invention relates generally to methods for measuring iron oxide levels in glass, and particularly to methods for correlating the iron oxide levels in glass with the oxidation state of that glass and further associating the oxidation state of the glass with the quality of the glass.
2. Technical Background
One of the major contributions to the manufacturing costs of glass articles continues to be the presence of defects in end-use glass articles. The presence of these defects in the manufactured glass articles can affect the suitability of the glass article for its application. These defects may be classified as solid in nature (e.g., unmelted raw materials, or other refractory materials) or gaseous in nature (e.g., bubbles or blisters). Chemical constituents are employed in the raw material mixtures to aid in the removal of these latter defects. These constituents are known as fining agents and encompass a range of multivalent oxide and/or halide materials. Typical fining agents are described, for example, in U.S. Pat. No. 5,824,127 (Bange) and U.S. Pat. No. 6,128,924 (Bange).
A relatively new, and demanding, use of glass is as a substrate for Flat Panel Displays (FPDs). One well-known member of this family of displays is the Liquid Crystal Display (LCD). In LCDs, two sheets of glass, each on the order of 1 mm thick, surround a thin, 3–5 μm, layer of liquid crystal material. Liquid crystal materials encompass a range of organic materials that have the properties of being birefringent (refractive index dependent upon the crystal orientation) and having an ability to switch crystal orientations by the application of an electric field. By applying polarizing films to the outside of this glass-liquid crystal glass “cell”, a voltage-controlled valve may be created. Pixelation necessary for a true display is done by applying the necessary voltage to only a small area of the total liquid crystal. This can either be done intrinsically (passive matrix Liquid Crystal Display) or actively through the creation of a thin film transistor (TFT) at each pixel location (Active Matrix Liquid Crystal Display, AMLCD). The addition of an external light source then completes the display.
The optical transmission characteristics of the glass substrate have a direct bearing on the optical performance of these FPDs, and in particular on the AMLCDs. A glass defect on the order of the pixel size may disrupt the optical transmission through that pixel and result in an optical defect in the display that is immediately evident to an observer/user of the display. Increases in both the display size and the display resolution have resulted in increasing demands placed on the glass manufacturer to deliver larger pieces of glass with smaller inclusions. A typical requirement for a glass substrate that will be used in AMLCDs is for the glass to contain no inclusion greater than about 50μm in an area of glass approximately 1 m×1 m.
To attain such high quality levels in the glass substrate, very tight control must be exercised over the entire glass manufacturing process. To minimize the occurrence of solid inclusions, tight control over the raw materials, melting system, and operation must be maintained. For gaseous inclusions, an additional level of control is incorporated in the manufacturing process, that is, the addition of a particular amount and type of fining agent(s), which is dependent on the efficiency of the fining agent. While a majority of the controls just discussed can be controlled in real time by standard operational controls, the efficiency of the fining agent has proven problematic. For fining packages based on multivalent oxides (e.g., As2O3, Sb2O3, SnO2, CeO2, Fe2O3), the efficiency is tied to the oxides' oxidation state.
The oxidation state of the glass has previously been determined to be reflected in the relative amounts of the iron ions Fe(II) and Fe(III) that exist as tramp components in most manufactured glass. Unfortunately, existing techniques for determining the iron oxidation states in the glass either require expensive and time consuming chemical analysis, for example, Atomic Absorption Spectroscopy (AAS) or Electron Paramagnetic Resonance (EPR) or require less accurate spectrophotographic methods (focused on two wavelengths to monitor a glass property, e.g., 300–400 nm and 1000–1200 nm), which although much quicker are often inconsistent. Therefore, either feedback time is slowed and/or quality of the feedback information is unreliable to the glass technologist or process engineer, making it difficult to make corrective actions within the glass melting process. As a result, there is a need in the industry to be able to accurately follow the iron ion levels, and thereby the oxidation states in the manufactured glass material. This is especially true for glass articles going to the highly demanding FPDs market. There is also a need in the industry for fast, efficient, and accurate methods to avoid producing and shipping unacceptable glass product, and where appropriate, a need to make this determination on site where the glass is manufactured, thereby minimizing the time it takes to make a corrective action. Against this backdrop the present invention has been developed.